The present invention relates to a method for anti-skid brake control, in which a car body speed is estimated from wheel speed information at braking, and a value relating to skidding of the wheels such as a wheel slipping rate is obtained from the estimated car body speed, and the braking is controlled according to this value, and in particular, it relates to an anti-skid brake control method for performing anti-skid brake control properly even when the car is steered during the anti-skid brake control.
Anti-skid brake control is performed as follows: In general, when it is detected that the wheels are in skid condition during braking, the braking force on the wheels is decreased to eliminate the skid condition. By increasing the braking force again thereafter, the steering of the vehicle is stabilized and braking distance is made as short as possible.
When this anti-skid brake control is performed, it is necessary to obtain a car body speed, which serves as a reference for the control, but it is difficult to find an accurate car body speed when the wheels are under skid condition during braking. Therefore, in performing anti-skid brake control, it has been normally practiced to estimate a car body speed, and anti-skid brake control is performed according to the estimated car body speed. In this case, various methods have been adopted to estimate the car body speed. One of the methods is to estimate the car body speed by Select-High. The method by Select-High is to obtain the estimated car body speed based on the highest wheel speed among the four front and rear wheels.
When preparing the estimated car body speed by the method of Select-High, if steering is performed during anti-skid brake control, it is possible that the speed of the wheel running at the outermost course becomes the highest. Thus, the estimated car body speed is prepared based on the wheel speed of the wheel running at the outermost course. In this case, the speed of outer rear wheel is usually much faster than the speed of the gravity center of the car body when the car is turning, and the estimated car body speed is obtained based on the speed of this outer rear wheel in most cases.
However, a slip ratio is obtained from the estimated car body speed, and the slip ratio thus obtained is higher than the actual slip ratio. Accordingly, if anti-skid brake control is performed with this slip ratio, brake cylinder pressure of each wheel is decreased too much, and this causes insufficient deceleration of the wheels. In particular, braking force is not sufficiently applied on the outer front wheel, which most urgently needs the braking force when the car is turning, and the braking force on the rear wheels is also not enough.